The invention in particular relates to an apparatus and method making use of high energy radiation such as x-rays or gamma-rays to scan objects where it is desirable to gain information about their internal contents and/or composition. This principle is widely employed for example in the security industry, but might also be employed in other areas, for example, without limitation, medical imaging, imaging for quality control purposes or the purposes of determining the integrity of the structure, or the like.
X-Ray absorption has been used as the basis for screening objects to create some form of representational image of the contents or components thereof relative to each other in three-dimensional space. The thicker or more dense an object is then the more it will attenuate an x-ray beam. By use of suitable detectors and a suitable source, radiographs of an item under screening in the form of images based on the absorption of an object or set of objects can be generated.
Typically, an x-ray source generates an essentially 2-dimensional beam and detectors of transmitted x-rays are used to build up successive image slices in cross-section based on transmitted x-rays (and hence differentiating by absorption). A computer is used to generate images of cross-sections of the object so they can be looked at one at a time. The cross-sections are then put together to form an image reflecting at least some three-dimensional cues. It is for example known to employ a line-scan principle, in which three dimensional objects are caused to move through a scanning zone and imaging information collected as it moves and an image built up from successive linear slices. It is also for example known to employ a computed axial tomography (CAT or CT) principle in which an image is built up from a series of two-dimensional images taken around a single axis of rotation. The precise way that an image might be generated from transmitted radiation is not pertinent to the present invention.
These known apparatus and methods tend to give limited information about the material content. In essence, at its simplest, all that is being measured is transmissivity of the object to the source radiation. The detector merely collects amplitude information, and does not discriminate transmitted radiation spectroscopically. In most practical systems even this is measured indirectly. At its simplest, a typical linear array x-ray detector comprises in combination a scintillator material responsive to transmitted x-rays, which is then caused to emit lower frequency radiation, and for example light in or around the visible region, in combination with a semiconductor detector such as a silicon or gallium arsenide based detector which is responsive to this lower frequency radiation.
However, it is known that spectroscopic information from transmitted x-rays could be used to give additional information about the material content of the objects or components being scanned. It is known that the x-ray absorption properties of any material can vary spectroscopically, and that the amount by which the absorption properties vary depends in particular on atomic number. This has led to development of dual-band or dual-energy detectors which are capable of separately identifying low- and high-energy bands from the full spectrum of x-ray emissions. Such a dual-energy sensor typically comprises a sandwich pair of semiconductor photodiode rays or the like, in conjunction with a low-energy and a high-energy scintillator, such that the respective detectors detect transmission of low-energy and high-energy x-rays. The differential absorption effect is exploited by the dual energy detector to differentiate generally between objects having lower and higher atomic number elements predominating.
When exploited as part of a security or like material identification system, a very crude approximation can be made that organic materials tend to be in the former category and most inorganic materials in the latter category. The practical implications of this have led to the use of such detectors in the security industry, and for example in airport x-ray scanners, either to create separate images of metallic items inside luggage (to reveal hidden metal items and especially weapons, such as guns, and knives) or to identify plastic explosives.
Most explosives are dense organic materials usually high in nitrogen content. There is therefore some limited merit in the use of dual energy detectors but it is far from being a precise explosive detector since many other items in luggage, such as soaps, creams, leather goods etc, are also dense organic materials.
A dual energy system confers only limited information about composition. The organic/inorganic division is crude and approximate. Conventional detectors do not give any real spectroscopic information about the spectrum of transmitted x-rays although they detect the presence or otherwise of x-rays within two distinct bands of the spectrum. Ultimately decisions are made based on the attenuation radiograph which is based on the shape of items and their proximity to other objects.
Recent development of detectors that can resolve spectroscopic information about the transmitted X-rays more effectively has led to the development of apparatus that discriminate across a larger range of bands and generate a larger plurality of images. For example U.S. Pat. No. 5,943,388 describes a system that makes use of cadmium telluride detectors to image across at least three energy bands and generate at least three images. Hamamatsu Photonics KK has developed a line sensor system under model number C10413 that makes use of cadmium telluride detectors to image across five energy bands. These better exploit the effect of differential spectral absorption by different materials and enable a better approximation to be made between transmissivity and composition.
Even with this resolution, such devices can still be confused by objects which are superimposed in the x-ray path, and will give no information concerning the crystalline or polycrystalline nature of an object.
Polycrystalline materials scatter x-rays and, the resulting x-ray image may hardly detect such polycrystalline material because a very large proportion of the X-rays which have not been absorbed by the material will have been scattered and so not received by the detector. This is unfortunate as in security x-ray screening a number of threat items are polycrystalline in nature, in particular plastic explosives such as CP4, RDX, PETN and proprietary formulations thereof, drugs and the like and are therefore difficult to detect by using conventional x-ray systems.
Crystalline or polycrystalline objects are capable of diffracting an x-ray beam if certain conditions are satisfied.
The situation is outlined using Bragg's Law which is:nλ=2d sin θWhere:                n is an integer (order of diffraction)        λ is the wavelength of the diffracted ray        d is the atomic lattice parameter        θ is the angle of diffraction        
At the specified wavelength (energy) the effect is close to 100%.
Attempts have been made to overcome detection problems associated with characteristic Bragg reflection by searching for the diffracted beam. If the threat material is specified then the information would be available concerning the diffraction angle θ0, the lattice parameter d, and the x-ray wavelength λ. In addition therefore, scanners have been proposed which make use of characteristic diffraction by including scatter detectors at appropriate scatter angles for particular target materials. Earlier patents GB2360685 and GB2329817 refer to just such an attempt. The energy of the diffracted photons is given by:
      E          p      ⁢                          ⁢      h        =      hc    λ  where h is Planck's constant and c is the speed of light.